The present invention relates generally to the inspection of load-bearing wire ropes and, more particularly, to continuous inspection of a wire rope having a free end such as a hoist rope used in rescue aircraft.
Continuous resistance monitoring of cables often involves transmitting one or more electrical signals through cables to calculate a resistance. As the cables wear, corrode, or are otherwise mechanically damaged, the resistance of the cables change. When the resistance exceeds a safe range, the cables are removed from service. Continuous resistance monitoring is particularly suited for load-bearing elevator cables. Elevator cables contain a series of parallel-extending and electrically-isolated cables, which permit the electrical signals to traverse the length of the cable. Each end of the elevator cable is anchored to a fixed structure, enabling the ends to be connected electrically to facilitate continuous resistance monitoring of the cable. However, conventional wire ropes are not similarly constructed.
Wire ropes of various sizes and configurations are constructed from a number of strands, each strand helically arranged about a metallic or non-metallic core. Each strand of the wire rope includes a number of wires, which like the strands, are arranged helically about a metallic or non-metallic center. Wire ropes are often wound around a hoist drum at a fixed end and affixed to a lifting hook or other mechanical attachment at the opposing free end. Because wires and strands are helically wound, the wires and strands contact each other along the length of the wire rope. The contact points are electrically-conductive, which short-circuit electrical signals sent from the fixed end of the wire rope and prevent the implementation of continuous inspection methods.
Some wire rope applications, for example conventional hoist ropes used in rescue aircraft, are used to lift loads that, should the wire rope fail, would result in extensive damage, serious injury, or loss of life. Since continuous resistance monitoring cannot be used, conventional wire ropes used in such critical applications are replaced after a predetermined time in service or in accordance with a recorded load history, which are not direct measurements of the mechanical condition of the wire rope. As a result, conventional wire ropes are often replaced prematurely. Therefore, a need exists for a wire rope capable of continuous resistance monitoring such that the wire rope can be replaced based on an actual mechanical condition of the rope.